Cardiac arrhythmias are a leading cause of morbidity and mortality within developed nations. Often, these arrhythmias are associated with acquired heart diseases, notably cardiac heart failure (HF), where the dysregulation of a host of ion transporters and channels is observed. Particularly, a critical imbalance of both depolarizing INa and repolarizing Ito is observed. Our previous work was the first to support the nascent idea that expression of INa and Ito may share common, yet to be identified regulatory mechanisms involving KChIP2, an accessory subunit of Ito. KChIP2 silencing produced simultaneous mRNA degradation and potential translational block of multiple genes at the source of INa and Ito, suggesting a mechanism of microRNA activity, which led to significant loss of Ito and INa. These results suggested KChIP2 may have additional nuclear functions as a transcription factor to regulate other critical cardiac currents. Indeed, a member of the KChIP family found in neuronal tissues, KChIP3, also known as DREAM, is localized to the nucleus where it acts as a Ca2+-regulated transcriptional repressor. Given KChIP2 shares a high degree of sequence homology with DREAM and that it has been found in the nucleus, one can hypothesize that KChIP2 is capable of similar nuclear roles in cardiac settings. Therefore, we hypothesize that KChIP2 controls expression of depolarizing INa and repolarizing Ito through a novel posttranscriptional mechanism involving microRNAs. Indeed, our preliminary data show evidence demonstrating KChIP2 transcriptionally regulates a family of miRNAs known as miR-34s which we demonstrate targets genes involved in generating both INa and Ito. However the significance of this pathway under pathologic conditions is unknown and is the central focus of this proposal. We hypothesize that KChIP2 loss in the diseased heart (hypertrophy and/or HF) is responsible for dysfunction of cardiac excitability at the level of gene expression. Given that KChIP2 is significantly repressed in HF, our specific aims are to: 1. Define the role of KChIP2 miRNA-dependent regulation in cardiac pathology. 2. Evaluate the influence of restored KChIP2 expression on arrhythmia susceptibility and HF progression. 3. Evaluate the influence of miRNA blockade by antagomirs on arrhythmia susceptibility and heart failure progression. To test these aims rat and guinea pig heart failure models produced by transverse aortic constriction (TAC) and the canine pacing-induced HF model will be used. Delivery of either KChIP2 through viral vectors or miR-34 blockade by injection of miRNA antagomirs will be used to assess influence over INa and Ito expression as well as overall arrhythmia susceptibility and HF progression. The delineation of the molecular basis of KChIP2 regulation is essential for an accurate understanding of cardiac depolarization and repolarization and its implications with lethal ventricular arrhythmias.